An ex-cop being sought in the weekend murders of a popular college couple was still at large after being involved in two shootouts with Los Angeles area police on Thursday, one of which resulted in the death of one officer.

Authorities began looking for Christopher Dorner, a 33-year-old former LAPD officer and Navy reservist, after discovering an online manifesto that contained threats against police officers and their families. He immediately became a suspect in what appears to be a revenge killing of the daughter of a former police captain and her fiancé.

Cal State assistant basketball coach Monica Quan, 28, and Keith Lawrence, 27, were found shot to death Sunday in a parked car in Irvine.

Police said Dorner opened fire on two LAPD officers in Corona on Thursday morning after he spotted them following him in a marked patrol car, according to local news station KTLA-TV. Dorner reportedly got out of his vehicle and began shooting at the officers with a rifle-like weapon. One officer suffered a graze wound and the other was not injured.

Shortly after, in Riverside, two more officers were ambushed. One was killed, and one was rushed to Riverside Community Hospital in critical condition, KTLA-TV said.

Dorner was believed to be driving a blue or gray Nissan Titan pickup, California license plate 8D83987 or 7X09131.

Authorities told the media Wednesday that they were searching for Dorner after reportedly recovering a multi-page online "manifesto," in which Dorner had made multiple threats against different people, including members of the LAPD.

"We have strong cause to believe Dorner is armed and dangerous," Irvine Police Chief David L. Maggard told CBS News on Wednesday, adding that the LAPD and FBI are assisting in the case and anyone who sees the suspect should immediately call 911.

Dorner was employed as a police officer from 2002 to 2009, when he was dismissed for making false statements against another officer. During a hearing in front of the Board of Rights, Dorner was represented by Monica Quan's father, Randal Quan, according to The Associated Press.

Randal Quan was the first Chinese American captain in the Los Angeles Police Department and later served as the police chief at Cal Poly Pomona, before practicing law.